One type of prior nonvolatile memory is the flash erasable and electrically programmable read-only memory ("flash EPROM"). The flash EPROM can be programmed by a user. Once programmed, the entire contents of the flash EPROM can be erased by electrical erasure. The flash EPROM may then be reprogrammed with new data.
Prior art personal computer systems typically employ removable data storage media. One common prior art removable storage medium is a floppy disk. A relatively new prior art storage medium is an integrated circuit-based memory card ("lC memory card").
Prior an flash EPROMs are nonvolatile and reprogrammable, and this has permitted the flash EPROM technology to be used for removable data storage. One such prior art application is the flash EPROM memory card ("flash memory card"). The flash memory card typically includes a number of flash EPROMs. The flash memory card can be erased and programmed electrically.
One type of prior flash EPROM used in the prior flash memory card typically includes redundant memory cells and CAM cells in addition to a main memory array. The redundant memory cells are used to replace defective cells of the main memory array. The redundant memory cells are also arranged into rows and columns and are therefore referred to as redundant memory array. When a memory cell in a column of the main memory array is of a prior flash EPROM found defective, a redundant column of the redundant memory array is used to replace the defective column in the main memory array.
The CAM cells are typically used to activate the redundant memory array to replace defective columns of the main memory array. The CAM cells typically comprise flash EPROM cells.
Disadvantages are, however, associated with the prior flash EPROM used in the prior flash memory card. One disadvantage is that when one prior flash EPROM has more defective cells than the redundant memory cells or the defective cells cannot be replaced by the redundant memory cells, this particular prior flash EPROM then cannot be used and has to be rejected. This typically causes the overall manufacture cost of the prior flash EPROM to rise significantly. Moreover, it is typically a waste to discard the entire flash EPROM because of a few defective memory cells within the memory array. This is typically the case when the storage capacity of the prior flash EPROM increases. Typically, the memory cells other than the defective memory cells of a flash EPROM can still be accessed for data storage.
Another disadvantage is that if the prior flash EPROM having unreplaced defective cells is used, the memory locations associated with these defective cells cannot be used and the addresses of those defective memory locations have to be taken out from the address map of the flash EPROM. This typically causes the address map for the flash EPROM to be non consecutive. When such a flash EPROM is used in a prior flash memory card, the addresses of those defective memory locations will also cause the card address map to be non consecutive. In addition, the external circuitry accessing the flash memory card needs to know those defective addresses prior to accessing the flash memory card.